216 



PROCEEDINGS OE THE GEOLOGICAL SOCIETY. [NOV. 16, 



ascertain still more completely if my general views respecting the 

 older rocks were correct, before I issued a small general geological 

 sketch-map of that region *, I now redeem my promise by offering the 

 following supplemental observations. 



As the classification- which I had proposed involved very consi- 

 derable changes in all preconceived ideas relating to the age of 

 many of the so-called primary crystalline rocks of the Highlands, I 

 naturally felt a strong desire that the Local Director of the Geo- 

 logical Survey of Great Britain, who was as much interested as 

 myself in adopting a correct view of the true order of nature, should 

 accompany me. 



The observations, therefore, which I now offer, flow from the joint 

 examination of the country by Professor Eamsay and myself, — 

 certain special improvements of the map of Sutherland, and the more 

 correct delineation of outlines of formations, being due to my fellow- 

 labourer, who spared no labour in ascending the highest summits 

 to determine the real relations of the rock-masses. 



The whole of this subject having undergone discussion at the late 

 meeting of the British Association, at Aberdeen, subsequently to my 

 last survey, and Professor Nicol having there expressed his dissent 

 from that essential part of the classification whereby I separate the 

 old or fundamental gneiss of the west coast from other flaghke 

 gneissose and micaceous rocks on the east, it has been a source of 

 satisfaction to me that Professor Harkness, who has since visited the 

 west of Sutherland to examine all the critical sections, has arrived 

 at precisely the same conclusions as Prof. Bamsay and myself, as 

 will appear in the sequel. 



Fundamental Gneiss. — In addition to the broad distinction formerly 

 pointed out between this massive and intensely crystallized old rock 

 and all the superior and younger rocks of gneissose character, 

 marked attention must be paid to the great discordance in the strike 

 or direction of these two rocks. 



The dominant strike of the fundamental gneiss of the north-west 

 coast is from N.N.W. to S.S.E., whilst that of the quartz-rocks, 

 limestone, and superior strata, whether micaceous or gneissic, is from 

 N.N.E. to S.S.W., as explained in this diagram (fig. 1). Again, look- 



Pig. 1. — Diagram-plan showing the general re- 

 lations of the Old or Fundamental Gneiss (a) 

 to the crystalline Lower /Silurian rocks (c and 

 d) where the intervening Cambrian rocks (b) 

 ■ are absent. 



a. Old gneiss. 



c, d. Quartz-rocks, limestones, mica-schists, and gneiss- 

 ose flagstones. 



This map (PI. 12. of vol. sv.) was published with No. 62 of this Journal. 



